The invention concerns a method for the separation of aromates from hydrocarbon mixtures of optional aromate content, mixtures which can contain as non-aromatic components, in particular, paraffins, cycloparaffins, olefins, diolefins as well as organic sulfur compounds. The separation occurs by means of extractive distillation in which N-substituted morpholine, the substitutions of which display no more than 7 carbon atoms, is employed as selective solvent, whereby the non-aromatic components of the hydrocarbon mixture, serving as an entry product are distilled off as raffinate across the top of the extractive distillation column. The aromates together with the employed solvent are discharged as extract from the sump of the extractive distillation column, and the raffinate is distilled for the purpose of recovery of the solvent residue present therein.
The above-described aromate recovery method has already been known for a number of years and has proven itself in practice, in various large-scale technical plants, particularly is the use of N-formylmorpholine as selective solvent. Herewith the sump product discharged from the extractive distillation column is normally led into a subsequently provided decanter, in which the aromates contained therein as extract are distillatively separated from the solvent. The solvent is then discharged from the sump of the decanter and reintroduced for repeat use in the extractive distillation column. Herewith the introduction and reintroduction of the solvent normally follows at the top of the extractive distillation column, for various process/technical reasons. However, it is practically unavoidable for the obtained raffinate to still contain a certain solvent residue whereby the solvent content in the raffinate can amount up to 2% by weight. For reasons of economy, and in view of a recovery of an as pure as possible raffinate, it is, however, indispensable to recover this solvent portion in the raffinate as extensively or completely as possible.
This would certainly be possible if one were to operate the extractive distillation columns with an appropriately high raffinate reflux. However, in contrast to normal distillation, such a reflux is unsuitable and, therefore, to be avoided with extractive distillation, for the following reasons:
1. A raffinate reflux leads to a dilution of the solvent and therewith to a decrease in selectivity, whereby the desired material separation is unnecessarily complicated. PA0 2. Highly selective solvents--and the above-mentioned N-substituted morpholine belong to this category--display only a limited dissolving behavior for the non-aromatic hydrocarbons to be separated. A raffinate reflux can therefore lead to the formation of two liquid phases with different densities at the upper plate of the extractive distillation column, which makes a disturbance-free operation of the extractive distillation column impossible.
Accordingly, this suggested manner for the recovery of the solvent portion of the raffinate is eliminated and instead a separate recovery of the solvent from the raffinate must take place. This was previously performed by means of simple distillation of the raffinate in such a manner that the raffinate is discharged as top-product from the distillation column with a solvent content of less than 10 ppm, whereas the solvent, concentrated to nearly 100% purity, is discharged from the sump of this column and then led back into the extractive distillation column.
This manner of operation, with which an as complete as possible separation of raffinate and solvent is desired, promotes, however, a high apparatus expenditure (i.e., distillation column with a high number of plates) and a high energy consumption.